This invention relates to spectroscopic analytical instruments and spectroscopic measuring instruments, and more particularly to monochromators suitable for spectroscopy with high resolution.
Plane grating monochromators have gained a wide application in the field of spectroscopic analysis, and the like. An example of a conventional Czerny-Turner monochromator which is believed to have relatively high resolution is shown in FIG. 2. Diverging beam 11 leaving an entrance slit 1 is converted to a parallel beam 12 by a spherical mirror 2. The beam 12 is converted to a monochromatic component beam 13 among the incident beams dispersed by a plane grating 6. The beam 13 is converted to a converging beam 14 by a spherical mirror 4 and passes through an exit slit 5. If the plane grating 6 is rotated at this time, the wavelength of the monochromatic beam passing through the exist slit 5 can be changed. The spherical mirror used hereby do not have the perfect collimation property of the diverging beam and perfect focusing property of the parallel beam that a paraboloidal mirror has. Therefore, each monochromatic spectral image converged onto the exist slit plane including exit slit and (a plane normal to the principal ray) has aberrations such as astigmatism, coma-type aberration, spherical aberration, and the like, and they impose limitations on the spectral characteristics of the monochromator.
The Czerny-Turner monochromator shown in FIG. 2 is discussed in Journal of the Optical of America, Vol. 54(1964), pp. 879-887. In this monochromator, the reduction of coma-type aberration in a predetermined wavelength region can be made by selecting the radii of curvature of the spherical mirrors or the incidence angles to them, and high resolution of the monochromator has been accomplished ordinarily in accordance with this method. However, this aberration reduction method is effective only in the proximity of a specific spectral wavelength to be dealt with, and has a small effect for the monochromator application which scans a wide wavelength region.
Recently, holographic grating whose grooves are formed by forming interference fringes by interference of laser beams and processing photographically the interference fringes has been put into practical application. In this case, aberration in the monochromator can be reduced by suitably selecting the arrangement of the interference fringes. "Proceedings of SPIE", Vol. 815 (1987), pp. 136-145 attains high resolution of a Czerny-Turner monochromator by producing a plane grating having varied-spaced and curved grooves by the interference of laser beams with an aspheric wavefront utilizing a hologram and a plane wavefront.
In a Littrow monochromator shown in FIG. 3, a spherical mirror 2 exhibits both the functions of collimation of the incident beam and focusing of the diffraction beam. However, greater coma-type aberration occurs in this case than in the CzernyTurner monochromator shown in FIG. 2, and high resolution cannot be obtained, though the optical configuration is simple.